LAPACK
3.4.2
LAPACK: Linear Algebra PACKage

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Functions/Subroutines  
subroutine  dlaqr1 (N, H, LDH, SR1, SI1, SR2, SI2, V) 
DLAQR1 sets a scalar multiple of the first column of the product of 2by2 or 3by3 matrix H and specified shifts. 
subroutine dlaqr1  (  integer  N, 
double precision, dimension( ldh, * )  H,  
integer  LDH,  
double precision  SR1,  
double precision  SI1,  
double precision  SR2,  
double precision  SI2,  
double precision, dimension( * )  V  
) 
DLAQR1 sets a scalar multiple of the first column of the product of 2by2 or 3by3 matrix H and specified shifts.
Download DLAQR1 + dependencies [TGZ] [ZIP] [TXT]Given a 2by2 or 3by3 matrix H, DLAQR1 sets v to a scalar multiple of the first column of the product (*) K = (H  (sr1 + i*si1)*I)*(H  (sr2 + i*si2)*I) scaling to avoid overflows and most underflows. It is assumed that either 1) sr1 = sr2 and si1 = si2 or 2) si1 = si2 = 0. This is useful for starting double implicit shift bulges in the QR algorithm.
[in]  N  N is integer Order of the matrix H. N must be either 2 or 3. 
[in]  H  H is DOUBLE PRECISION array of dimension (LDH,N) The 2by2 or 3by3 matrix H in (*). 
[in]  LDH  LDH is integer The leading dimension of H as declared in the calling procedure. LDH.GE.N 
[in]  SR1  SR1 is DOUBLE PRECISION 
[in]  SI1  SI1 is DOUBLE PRECISION 
[in]  SR2  SR2 is DOUBLE PRECISION 
[in]  SI2  SI2 is DOUBLE PRECISION The shifts in (*). 
[out]  V  V is DOUBLE PRECISION array of dimension N A scalar multiple of the first column of the matrix K in (*). 
Definition at line 122 of file dlaqr1.f.